Earlier quoted context omitted.
Endogenous factors are addressed in the paper. Nothing personal, but I'm getting so tired of people on HN saying 'well, maybe they made an elementary statistical mistake...' They're professional statisticans working with a large and robust dataset,a nd they devoted 6-7 pages of the paper to discussing how they controlled for such factors. And they brought up your exact point in the very first paragraph of that discus…
I agree that we should always at least read the linked article (and sometimes the original paper) before bringing out "correlation does not equal causation". However, you are overstating the case for the author's having dealt with reverse causality. From the conclusion: When we addressed causality in IV estimates, the significance of the correlation between participation in social networking sites and subjective well…
My gripe is with reflexive and unjustified statistical one-upmanship in HN discussions.